The cluster glycoside effect describes the remarkably high affinities of polyvalent saccharide ligands for polyvalent lectins, as measured by agglutination assays. Because of the myriad roles played by protein- carbohydrate rate interaction in the earliest phases of human disease, the notion of polyvalent saccharide ligands as human therapeutics has gained prominence during the last several years. Although the cluster glycoside effect is well described phenomenologically, the molecular origin of the effect is not at all clear. More significantly, the applicability of polyvalent ligands for in vivo therapeutic use remains questionable, since those phenomena responsible for the effect in vitro may not be applicable in vivo. This project will explore the molecular origin of the cluster glycoside effect. Specifically, we will: 1. Prepare a variety of spatially defined homogenous polyvalent glycosides for studying multivalency effects in protein-carbohydrate interaction. 2. Develop combinatorial libraries of peptide-linked bivalent carbohydrate ligands for rapid screening of large numbers of compounds. 3. Utilize titration microcalorimetry to determine the molecular origin of the cluster glycoside effect. 4. Study both the crystal and solution structures of high affinity ligands, to determine structural features of polyvalent ligands relevant to tight binding. We will carry out our experiments in the context of the shiga-like toxin. The compounds developed here would potentially be useful therapeutic compounds for the treatment of human infection by E. coli 0157.